URI resolution chokes on URI scheme not supported by JVM

I am using Calabash with a custom `URIResolver` that resolves URIs of the `site` scheme (e.g. `site:/pages/index.html`).

Unfortunately, Calabash's `XProcURIResolver` tries to construct a `java.net.URL`  from the URI, and since the running JVM doesn't have a handler for the `site` scheme, this fails, even though the custom `URIResolver` would translate it into something usable.

This is a patch for that problem. Now, the `MalformedURLException` is caught and ignored, and the unaltered input URI used if a `java.net.URL` could not be constructed. I'm not familiar with the `jar` URI use case your code there is meant to address, but I'm hoping this change won't break that use case.